1. Field of the Invention
The present invention relates generally to satellite communication systems and networks. More particularly, the present invention relates to quickly and accurately determining the position of a user terminal in a satellite communications system.
2. Related Art
A typical satellite-based communications system comprises at least one terrestrial base station (hereinafter referred to as a gateway), at least one user terminal (for example, a mobile telephone), and at least one satellite for relaying communications signals between the gateway and the user terminal. The gateway provides links from a user terminal to other user terminals or communications systems, such as a terrestrial telephone system.
While the advent of the wireless user terminal has freed users to take their phones wherever they go, it has also imposed upon the satellite communications carrier the burden of determining the location of the user terminal. The need for user terminal position information stems from several considerations. One consideration is that the system should select an appropriate gateway for providing the communications link. One aspect of this consideration is allocation of a communications link to the proper service provider (for example, a telephone company). A service provider is typically assigned a particular geographic territory, and handles all communication links or calls with users in that territory. When communications are required with a particular user terminal, the communications system can allocate the call to a service provider based on the territory within which the user terminal is located, at the time. In order to determine the appropriate territory, the communications system requires the position of the user terminal. A similar consideration arises when calls must be allocated to service providers based on political boundaries or contractual relationships.
Several methods of determining the location of the user terminal involve measuring the distance (range) between the user terminal and the satellite and the rate of change of that distance (range rate). When these measurements are combined with other data, such as the position of the satellite, the location of the user terminal can be determined with great precision. Techniques for determining user terminal location using satellite-user terminal range and range rate are disclosed in commonly-owned, copending U.S. patent applications entitled "Position Determination Using One Low-Earth Orbit Satellite," Ser. No. 08/723,751; and "Unambiguous Position Determination Using Two Low-Earth Orbit Satellites," Ser. No. 08/723,725, which are incorporated by reference herein in their entirety.
Once the range and range rate quantities are known, they can be combined with the known position and velocity of the satellite to generate a set of non-linear equations that cannot be solved exactly. According to conventional methods, a solution to these equations is obtained by using a numerical approach, such as the well-known iterative Newton-Raphson method.
One disadvantage of this approach is that such iterative numerical approaches require significant time and computational resources to execute. A crucial requirement in position determination for a satellite-based communications system is speed. When communications are required with a particular user terminal, the position of the user terminal should be selected rapidly. Since user terminal position is used as part of the process of establishing service, delays in determining position result in undesirable delays in establishing service. A mobile terminal user is not likely to tolerate a delay of more than a few seconds when placing a call or setting up a communications link.